1. Field of the Invention
The present invention relates to a method of forming conductive elements on thin-film semiconductor devices. "Thin-film" semiconductor devices generally are understood to comprise layers of material each less than 10 micrometers (100,000 .ANG.) thick successively fabricated on a flat substrate. More particularly, the present invention relates to a method of forming back electrodes on photovoltaic cells comprised of thin films of amorphous silicon.
2. Description of the Related Art
As is well known in the thin-film semiconductor art, photovoltaic cells that convert solar radiation into usable electrical energy can be fabricated by sandwiching certain semiconductor structures, such as, for example, the amorphous silicon PIN structure disclosed in U.S. Pat. No. 4,064,521, between two electrodes. One of the electrodes typically is transparent to permit solar radiation to reach the semiconductor material. This "front" electrode can be comprised of a thin film (i.e., less than 10 micrometers in thickness) of transparent conductive oxide material, such as tin oxide, and usually is formed between a transparent supporting substrate made of glass or plastic and the photovoltaic semiconductor material. The "back" electrode, which is formed on the surface of the semiconductor material opposite the front electrode, generally comprises a thin film of metal such as, for example, aluminum.
The voltage produced across the electrodes of a single photovoltaic cell, however, is insufficient for most applications. To achieve a useful power level from photovoltaic semiconductor devices, individual photovoltaic cells must be electrically connected in series in an array referred to herein as photovoltaic "module." A typical arrangement of series-connected photovoltaic cells is shown in FIG. 1.
FIG. 1 shows photovoltaic module 10 comprised of a plurality of series-connected photovoltaic cells 12 formed on a transparent substrate 14 and subjected to solar radiation 16 passing through substrate 14. Each photovoltaic cell 12 includes a front electrode 18 of transparent conductive oxide, a photovoltaic element 20 made of a semiconductor material, such as, for example, hydrogenated amorphous silicon, and a back electrode 22 of a metal such as aluminum. Photovoltaic element 20 can compromise, for example, a PIN structure. Adjacent front electrodes 18 are separated by first grooves 24, which are filled with the semiconductor material of photovoltaic elements 20. The dielectric semiconductor material in first grooves 24 electrically insulates adjacent front electrodes 18. Adjacent photovoltaic elements 20 are separated by second grooves 26, which are filled with the metal of back electrodes 22 to provide a series connection between the front electrode of one cell and the back electrode of an adjacent cell. Adjacent back electrodes 22 are electrically isolated from one another by third grooves 28.
The thin-film photovoltaic module of FIG. 1 typically is manufactured by a deposition and patterning method. One example of a suitable technique for depositing a semiconductor material on a substrate is glow discharge in silane, as described, for example, in U.S. Pat. No. 4,064,521. Several patterning techniques are conventionally known for forming the grooves separating adjacent photovoltaic cells, including silkscreening with resist masks, etching with positive or negative photoresists, mechanical scribing, electrical discharge scribing, and laser scribing. Laser scribing and silkscreening methods have emerged as practical, cost-effective, high-volume processes for manufacturing thin-film semiconductor devices, including amorphous silicon photovoltaic modules. Laser scribing has an additional advantage over silkscreening because it can separate adjacent cells in a multi-cell device by forming separation grooves having a width less than 25 micrometers, compared to the typical silkscreened groove width of approximately 380-500 micrometers. A photovoltaic module fabricated with laser scribing thus has a larger percentage of its surface area actively engaged in producing electricity and, consequently, has a higher efficiency than a module fabricated by silkscreening. A method of laser scribing the layers of a photovoltaic module is disclosed in U.S. Pat. No. 4,292,092.
Referring to FIG. 1, a method of fabricating a multi-cell photovoltaic module using laser scribing comprises: depositing a continuous film of transparent conductive oxide on a transparent substrate 14, scribing first grooves 24 to separate the transparent conductive oxide film into front electrodes 18, fabricating a continuous film of photovoltaic semiconductor material on top of front electrodes 18 and in first grooves 24, scribing second grooves 26 parallel and adjacent to first grooves 24 to separate the semiconductor material into individual photovoltaic elements 20 and expose portions of front electrodes 18 at the bottoms of the second grooves, forming a continuous film of metal on elements 20 and in second grooves 26 so that the metal forms electrical connections with front electrodes 18, and then scribing third grooves 28 parallel and adjacent to second grooves 26 to separate and electrically isolate adjacent back electrodes 22.
Complete reliance on laser scribing to pattern photovoltaic modules in the manner described above, however, heretofore has not been practical. The reflectivity of the metal forming back electrodes 22 requires use of relatively high laser power densities during scribing of third grooves 28, which has been found to damage the underlying semiconductor material of photovoltaic elements 20. When the photovoltaic elements are comprised of amorphous silicon, the damage resulting from laser scribing the overlying metal includes recrystallization of the amorphous silicon. Such recrystallization tends to create electrical connections between adjacent back electrodes, which produces short circuits between paired front and back electrodes and substantially reduces the efficiency of the photovoltaic module. Shorting also can result from the laser causing the back electrode metal to diffuse into the underlying semiconductor material to form conductive alloys. Consequently, in prior art patterning methods, silkscreening with acid etching must generally be used to form the grooves separating the back electrodes and to produce an operable photovoltaic module.
The present invention is intended to provide a method of laser patterning the metal film forming the back electrodes on thin-film semiconductor devices, including amorphous silicon photovoltaic devices, without damaging the semiconductor material underlying the metal film. Such a method would provide distinct advantages over conventional silkscreen patterning methods.
For instance, a photovoltaic module having laser-patterned back electrodes can be provided with a larger active area by taking advantage of the ability to form narrower grooves in thin-film devices by laser scribing than by acid etching with silkscreened etch resists.
In addition, patterning back electrodes by silkscreening requires additional processing steps that significantly decrease production throughput and increase labor costs relative to a laser patterning process. For example, etching third grooves 28 to separate back electrodes 22 of photovoltaic module 10 requires the steps of (1) forming a silkscreened pattern of acid-resistant material over the metal film to cover the back electrodes and expose the desired dividing lines between the back electrodes, (2) etching the exposed portions of the metal with acid to form third grooves 28, (3) rinsing the photovoltaic module to remove the acid, (4) applying a solvent to remove the silkscreened pattern, (5) rinsing the photovoltaic module to remove the solvent, and (6) drying the photovoltaic module.
Furthermore, a practical method of laser patterning back electrodes on a thin-film photovoltaic module without damaging the semiconductor film, when combined with conventional methods of laser scribing the conductive oxide film and photovoltaic semiconductor film, eliminates any need for silkscreen processing equipment and personnel in the manufacture of thin-film photovoltaic modules.
The present invention also is intended to provide an improved method of forming the electrical interconnections between the front and back electrodes of a photovoltaic module separated by a thin film of semiconductor material. In prior art methods, the interconnections are formed by scribing grooves in the semiconductor material to expose portions of the front electrodes at the bottoms of the grooves and then fabricating a metal film over the scribed semiconductor film and in the grooves. Thus, the module must be transferred from a semiconductor film deposition station to a laser scribing station and then to a metal film deposition station. One embodiment of the present invention is intended to provide a method of forming the interconnections between front and back electrodes that eliminates the laser scribing step conventionally performed between deposition of the semiconductor film and metal film and creates the interconnections at the same time as the grooves separating the back electrodes are formed. By forming the interconnections simultaneously with the grooves separating the back electrodes, further increases in production efficiency are obtainable.
Additional advantages of the present invention will be set forth in part in the description that follows and in part will be obvious from that description or can be learned from practice of the invention. The advantages of the invention can be realized and obtained by the method particularly pointed out in the appended claims.